Dew formation

Moisture precipitation Apart from wetting by sea-spray, moisture may either be deposited on a surface by rainfall or dew formation. For a known ambient humidity the dew point can be calculated, using the expression given previously, from standard tables giving the saturated vapour pressure of... 

An improvement on the chilled mirror is the cycled chilled-mirror-type dew point detector. The advantages of cycling the mirror temperature include simplicity, which also reduces the cost and lowers maintenance, because, by making the periods of dew formation short (5% of the time), the probability of contaminant condensation on the mirror surface is also reduced. In this design, the mirror surface is visually monitored by a fiber-optic link. 

At 10°C, a layer of liquid water 0.5 mm thick corresponds to the content of water vapor in a column of saturated air (see Appendix I) that is 53 m tall. Hence, considerable downward movement of water vapor is necessary to have dew formation over large areas, or much of the water vapor must emanate from the ground. 

Incubation of the host tissue after inoculation with a plant pathogen or bioherbicide is the most in rtant factor determining success (efficacy) of the inoculation (3il). when velvetleaf is inoculated with F. lateritium spores (10 spores/ml) harvested from petri plates (31.32), incubation in an environment conducive to dew formation is recjuired ixmnediately after inoculation (Figure 4). A delay of 4 h from host inoculation until the occurrence of dew resulted in a 50% decline in efficacy of this bioherbicide. 

Sterox NJ, AG-98) or emulsified oils 1% (v/v) (Agri-Dex, Soy-Dex) on efficacy of Alternaria cassiae (1.1 kg/ha) under field conditions. Sicklepod plants in cotyledon to 1-leaf growth stage were inoculated with A. cassiae spores in 337 L/ha. No dew formation occurred for 24 h after inoculation. Overhead irrigation (0.6-1.3 cm) was applied to plots 24 h after inoculation. Observations were recorded 4 weeks after treatment. Spores were produced by a proprietary modification of the method of Walker (15). ... 

Atmospheric Concentrations. Atmospheric concentrations of the major depositing species were determined during the artificial dew formation period. The species measured were NO, NO2, Oo, HNOo, SO2, HCl, NH, and particulate NO3, S0 J, Cl , NHjJ, Ca, Mg Na, and K. ... 

With this outlook, the key factors appear to be the coincident occurrence of SO2 and moisture. The controlled metal exposures from the ASTM studies were of flat panels in standard orientation. The simple geometry avoids many complications from moisture retained in surface texture features. Therefore, as a first approximation, we take the relevant time of wetness to be short periods following precipitation and periods of dew formation. The formation of a film of water is most likely at high relative humidities high humidities are most prevalent during the night hours. In the eastern U.S., summer predawn humidity is typically 80-90% compared to afternoon humidities of 50-60% winter patterns are less pronounced in areas north of the Ohio river, but also show afternoon minima (3) ... 

The coefficients in Table 3 provide a better theoretical understanding of how the different factors affect the corrosion of weathering steel. The 3 coefficients affect the solubility of the protective oxide layer and the a coefficients affect the diffu-sivity through the layer. The large ratio of a/3 confirms the relative insolubility of the rust on weathering steel in most environments. Sulfur dioxide increases the solubility of the film while NO2 and rain decrease the solubility. Rain apparently washes away acidic components (deposited during dew formation) that increase the solubility. 

During wet periods, the gas phase concentrations decreased due to uptake by the panels. The time dependence of the gas phase concentration of species x during a period of dew formation can be approximated by the following equation ... 

Relative humidity leads to the formation of a thin surface film on a metal when exposed to rain, fog, or dew formation. Xu et al. [52] monitored the dew formation process with a specially designed experimental arrangement. The results indicated that dust on the metal surface facUitates dew formation and increases atmospheric corrosion. Atmospheric corrosion requires both a thin film as well as some type of contaminant to initiate. The process of dew formation occurs much more rapidly in the presence of salts on the metal surface. Figures 10.13 and 10.14 compare the development and advancement of the dewing process on a clean and dust-contaminated surface, respectively [52]. Dew formation was monitored on mild steel (i) before dewing (ii) after 2 min (iii) after 4 min (iv) after 8 min and (v) after surfece drying (ambient temperature 13 °C, relative humidity 69). If the d.c. current for... 

Fig. 10.13 Dewing formation on miid steel surface taken at different moments (a) before dewing (b) after 2 min (c) after 4 min (d) after 8 min (e) after surface drying (ambient temperature 13 °C, reiative humidity 69%) [52].

The washing effect, which occurs with rain, is usually slight or negligible. With little or no run-off, the pollutants remain in the electrolyte and continue their corrosive action. As the dew dries these contaminants remain on the surface to repeat their corrosive activity with subsequent dew formation. 

Dew formation occurs when the temperature of the metal surface is below the dew point of the atmosphere. This may occur outdoors during the night, when the surface temperature may decrease by radiant heat transfer between the structure and the sky. Another reason for dew formation may be the conditions in the early morning, when the temperature of the air increases faster than the temperature of the metal, especially if the mass, and thus the heat capacity, of the metal is great. Dew may also form when metal products are brought into warm storage after cold transport. 

For nearly horizontal surfaces in particular, wetting time is determined less by rain than by nightly dew formation that can cause intensive wetting on about two out of three nights, even when it has not rained for days. Such surfaces are wet for approximately half of the year [87]. 

Pyatt (1969a) carried out additional experiments by enclosing lichen thalli in petri dishes containing water to maintain a humid atmosphere. In some cases droplets of water were pipetted and positioned over ascocarps, i.e., synthetic dew formation. In other cases the thalli were flooded. From Table VII it can be seen that these forms of precipitation do encourage sporulation. 

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